Super rodents build super habitat

Faster than a speeding coyote, able to leap small
cacti in a single bound -- two “superhero” rodents, the
kangaroo rat and the prairie dog, thrive amid the heat and dry sand
of the desert Southwest. Each creature influences its environment
to an extent that far outweighs its size – a real-life
version of Mighty Mouse. In the Chihuahuan Desert, banner-tailed
kangaroo rats and Gunnison and black-tailed prairie dogs fill the
role of keystone species, animals whose presence significantly
affects the overall health of an ecosystem. Now, a groundbreaking
new study has determined that where one keystone species is good,
two may be even better. For with their powers combined, these
“superhero” rodents create super habitat.

The
Chihuahuan Desert is North America’s second largest, 140,000
square miles spanning Arizona to Texas and down into Mexico,
encompassing an area almost as large as California. In this desert,
“sky islands” rise from a brittle and windblown basin,
stubbly with prickly pear, agave and grasses. These mountains
support pockets of temperate climate and create one of the most
diverse desert habitats in the world.

Here, the presence
of ecosystem engineers, such as kangaroo rats and prairie dogs,
adds to the desert’s diversity. These rodents dig burrows and
disperse seeds, providing shelter and food for hundreds of other
animals. The changes trigger ripple effects that cascade throughout
all levels of the ecosystem, influencing everything from top
predators, such as sharp-eyed eagles, to chemical processes, such
as nutrient cycling. And while a solitary kangaroo rat burrow or a
prairie dog colony can harbor a Noah’s Ark of other critters,
researchers at the University of New Mexico have found that a
combination of the two types of burrowers boosts the overall health
of the environment by significantly increasing the diversity of
habitat and wildlife. “We’re looking at two ecosystem
engineers that have really large transformation effects on the
habitat,” says Ana Davidson, who completed the study for her
doctoral thesis at the University of New Mexico.

Between
2000 and 2002, Davidson and associate professor David C. Lightfoot
monitored invertebrates around kangaroo rat and prairie dog
colonies. They collected and identified more than 250 species of
insects, spiders and other invertebrates relying on the habitat
created by these small rodents. “I wanted to know, is a mound
just a mound and is a burrowing rodent just a burrowing
rodent?” says Davidson.

What she found was that
while kangaroo rats and prairie dogs are similar in some ways --
burrowing, creating mounds of dirt at tunnel openings and feeding
primarily on grasses, seeds and wildflowers -- they nevertheless
play distinct roles within their environment.

Not only
are those roles distinct, they’re synergistic; where prairie
dogs and kangaroo rats co-exist, a healthier environment prevails.
For instance, the researchers found that in central New
Mexico’s Sevilleta National Wildlife Refuge and
Mexico’s Janos-Casas Grandes region, insects, spiders and
other invertebrate populations have surged in areas where both
rodents live. A plethora of insects backpack on the hard work of
the two rodents as they diversify plant communities and excavate
their subterranean cities.

“It was a surprisingly
strong pattern,” says Davidson, referring to the increase in
abundance and variety of bugs connected with the multi-rodent
metropolis. Her surveys indicate a greater than 95 percent
certainty that the relationship between the rodents and
invertebrate communities is not due to random chance.

Differences in prairie dog and kangaroo rat behavior create the
bonanza of habitat and food choices for other animals.

Kangaroo rats selectively harvest and store seeds underground, a
ready food cache for raiding beetles and a rare burrowing ant. The
rodents scatter seeds across the desert when they gather and hoard,
shaping plant communities as they scurry about. Prairie dogs, on
the other hand, rarely bring food into the burrow, grazing instead
on open prairie. They eat different plants than the kangaroo rat
does and act almost like a natural lawn-care service for the
grasses and wildflowers around their mounds. This
“gardening” supports a variety of grasshoppers and
crickets, which rely on everything from barren soil to dense
patches of grass.

The rodents’ subterranean homes
offer a variety of shelter for insects, spiders and other
invertebrates. Kangaroo rats excavate intricate mazes, in which the
temperature fluctuates only moderately throughout the day, creating
a more stable habitat than at the desert surface. These tunnels are
particularly appealing to ground-dwelling invertebrates, such as
beetles and spiders. Prairie dogs dig deep, long tunnels that are
damp and cool. It’s the antithesis of the desert -- just
right for camel crickets and burrowing ants, elusive inhabitants of
desert environments. Keystone species create biological
“hotspots,” says Davidson, “and you find these
rare species that would not have homes if it weren’t for
these engineering rodents.”

The increased diversity
of insects supported by the rodents means more scavengers are
working to decompose plant matter and dead animals, cycling
nutrients through the system. Insect-eating birds feed on the bugs,
and other predators, such as rattlesnakes and the Mexican gray
wolf, prey on the birds, kangaroo rats and prairie dogs. It’s
a cycle anchored by the changes wrought by these dynamic rodents.

Davidson and Lightfoot’s project ranks high on the
“cool” meter, according to other scientists. Their
research verifies what ecologists have suspected would happen
should two keystone animals share the habitat. Scott Mills, a
population ecologist with the University of Montana, says the
majority of wildlife species in most environments are not
considered keystone species and play only moderate roles in shaping
the ecosystem. But when two or more keystone species are present,
their effects are likely to cumulate, with each species strongly
influencing the ecosystem in distinct ways. “It definitely
gives a good, strong message for conservation,” says Mills.
“Losing one (of these keystone) species could start popping
the proverbial rivets off the airplane.”